Need to insulate existing dormer roof/ceiling, which way ?

[BuggBairn]

Last year we moved into a 1960's bungalow with the loft space converted with a rear full length dormer which encapsulates 3 bedrooms, an En-suite and a family bathroom, the work was done late 1970's.

Unbeknown to us the flat roof appears to lack any sort of decent insulation. We've managed to knock up an £870 underspend on our heating bill for the last quarter If we leave the heating off overnight the upstairs cools very quickly.

In an ideal world I'd like to create a "floating ceiling" underneath the current one using batons, PIR board and foil backed plasterboard.

My main issue appears to be the windows. Where the top of the window recess ends there is only a timber facing of approx 100mm then the ceiling starts.

I'm not sure cosmetically how that would look ie, no space between the window recess and the ceiling.

Would I have future issues with mounting of curtain poles/blinds without the window facing ?

The only other two options I can think of would be to:-

1. Tear our existing ceilings, insulate above the replace.

2. Create a sort of step in the ceiling and end the newly insulated ceiling early where it approached the window, say 1 foot away.

Both have their drawbacks as far as I can see.

1. Disruption/mess - we have an 18 month old kid and another arrival in June.

2. How would it end up looking cosmetically and would there be a lot of heat loss in the "last foot" area at the window that has not been insulated.

I'd like folks opinions on these matters and possible ideas for an alternative to what I'm suggesting/pondering. I intend to do as much of the work myself and leave the finishing to the experts.

thanks for looking

 

[ree]

Google pics and section diagrams of warm and cold flat roofs.

Determine which roof is yours, and then think through the various difficulties you might be creating for yourself with your proposal.

A 1970's flat roof (depending on materials) might be due for a tear off, and custom renewal.

 

[BuggBairn]

The roof covering itself has been replaced recently so right now it makes no financial sense to look at warm roof designs.

Having read up on Cold roof / warm roof insulation techniques I think I'm going to opt for tearing down the existing ceiling and starting a cold roof insulation construction from under the deck from scratch.

I won't know what level of clearance I have until I remove the existing ceiling, I'm hoping the roof joists are at least 150mm deep, that'll give me enough room to create a 50mm ventilated void and have 100mm of Kingspan/celotex PIR board fitted.

 

[BuggBairn]

Okay

I've started investigating what ceiling insulation this room has and there's Hee-Haw, nowt, not a bit of isulation, just a void between the deck and plasterboard. That'll explain the underpayment on our last Gas Bill .

The good news is that there is a 170mm gap at the narrow end which increases to 220mm as the flat roof rises slightly.

My plan now is to use this void to it's max and fit 120mm PIR Board leaving a 50mm ventilated space above.
I plan on finishing the ceiling with plasterboard that also has 13mm of insulation backing on it.

Does my set up sound okay ?

 

[BuggBairn]

Those wooden straps that are fitted between joists to catch the edges of your ceiling plasterboard. What do folks do with them when you're fitting a thick sheet of king span ?

Edit. I believe they are noggings I'm referring to.

 

[tony1851]

Okay

I've started investigating what ceiling insulation this room has and there's Hee-Haw, nowt, not a bit of isulation, just a void between the deck and plasterboard. That'll explain the underpayment on our last Gas Bill .

The good news is that there is a 170mm gap at the narrow end which increases to 220mm as the flat roof rises slightly.

My plan now is to use this void to it's max and fit 120mm PIR Board leaving a 50mm ventilated space above.
I plan on finishing the ceiling with plasterboard that also has 13mm of insulation backing on it.

Does my set up sound okay ?

Don't forget a vapour barrier at the level of the underside of the rafters

 

[BuggBairn]

Do I need a vapour barrier if I have a ventilated space of 50mm under the roof deck ? I was under the assumption that would take care of any possible condensation.

 

[tony1851]

Do I need a vapour barrier if I have a ventilated space of 50mm under the roof deck ? I was under the assumption that would take care of any possible condensation.

Yes, you could well find that condensation forms on the top surface of the kingspan and dribbles down to where the wall meets the roof.

In winter particularly, the amount of moisture formed cannot easily be removed by diffusion through the breather felt.

Personally, I wouldn't risk leaving out a vapour barrier.

 

[BuggBairn]

I won't be taking any risks on condensation as this will be our newborns room come June.

I've ordered up a 1200 gauge membrane and will staple it on the underside of the joists and noggins.